The invention relates to a process for the preparation of middle distillates from a mixture of carbon monoxide and hydrogen.
The preparation of hydrocarbons from an H.sub.2 /CO mixture by contacting said mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropsch hydrocarbon synthesis process. Catalysts frequently used for this purpose contain one or more metals of the iron group together with one or more promoters and sometimes a carrier material. The preparation of Fischer-Tropsch catalysts can in principle be carried out by any of three methods, namely by precipitation, melting or impregnation. Both the precipitation route and the melting route are not very attractive preparation methods for the Fischer-Tropsch catalysts since their reproducibility is low. The precipitation route is moreover very time-consuming, and the melting route requires much energy. Also, the catalytic properties of the catalysts prepared by melting and precipitation, particularly activity and stability, are often not entirely satisfactory. A much more attractive preparation method for the Fischer-Tropsch catalysts is the impregnation route. This is simple to carry out, yields well-reproducible results and generally leads to catalysts having high activity and stability.
Applicants have carried out an extensive investigation into the preparation of hydrocarbons from H.sub.2 /CO mixtures using Fischer-Tropsch catalysts prepared by impregnation. This investigation revealed that the behavior of these catalysts in said conversion greatly depends on the following factors:
(1) the nature of the metal of the iron group and the load used, PA1 (2) the nature of the promoter and the load used, PA1 (3) the nature of the carrier and PA1 (4) the temperature treatment used. PA1 (a) thermally stable to a temperature above 600.degree. C., PA1 (b) an X-ray powder diffraction pattern containing the four lines stated in Table A as strongest lines. PA1 (c) in the formula representing the composition of the silicate expressed in moles of oxides and containing in addition to oxides of hydrogen, alkali metal and/or alkaline earth metal and silicon, one or more oxides of a trivalent metal A chosen from the group consisting of aluminum, iron, gallium, rhodium, chromium and scandium, the SiO.sub.2 /A.sub.2 O.sub.3 molar ratio is more than 10. PA1 (a) catalysts containing 30-75 parts by weight of iron and 5-40 parts by weight of magnesium per 100 parts by weight of alumina and prepared by impregnating an alumina carrier with one or more aqueous solutions of salts of iron and of magnesium followed by drying the composition, calcining at a temperature of 700.degree.-1200.degree. C. and reducing it. Particularly preferred are catalysts of this type that contain in addition to 40-60 parts by weight of iron and 7.5-30 parts by weight of magnesium, 0.5-5 parts by weight of copper as reduction promoter and 1-5 parts by weight of potassium as selectivity promoter per 100 parts by weight of alumina and which are calcined at 750.degree.-850.degree. C. and reduced at 250.degree.-350.degree. C. PA1 (b) Catalysts containing 10-40 parts by weight of iron and 0.25-10 parts by weight of chromium per 100 parts by weight of silica and prepared by impregnating a silica carrier with one or more aqueous solutions of salts of iron and of chromium followed by drying and calcining the composition and reducing it at a temperature of 350.degree.-750.degree. C. Particular preference is given to catalysts of this type that contain in addition to 20-35 parts by weight of iron and 0.5-5 parts by weight of chromium, 1-5 parts by weight of potassium as selectively promoter and which are calcined at 350.degree.-700.degree. C. and reduced at 350.degree.-500.degree. C.
It was further found that the use of these catalysts usually results in a product having a very broad molecular weight distribution, however, only a small part of said product consists of middle distillates. In addition to the yield, the pour point of said middle distillates is also unsatisfactory. In this connection the direct conversion of H.sub.2 /CO mixtures by the Fischer-Tropsch process is a not very attractive route for the preparation of middle distillates on a technical scale.
Further investigation by the Applicants into the preparation of middle distillates from H.sub.2 /CO mixtures revealed that a valuable product for middle distillate is obtained with the use of certain catalysts.